Interplay between insulin signaling, juvenile hormone, and vitellogenin regulates maternal effects on polyphenism in ants Romain Libbrechta,b,1, Miguel Coronaa,c, Franziska Wended, Dihego O. Azevedoe, Jose E. Serrãoe, and Laurent Kellera aDepartment of Ecology and Evolution, University of Lausanne, CH-1015 Lausanne, Switzerland; bLaboratory of Insect Social Evolution, Rockefeller University, New York, NY 10065; cBee Research Laboratory, US Department of Agriculture–Agricultural Research Service, Beltsville, MD 20705; dDepartment of Animal Ecology I, University of Bayreuth, D-95440 Bayreuth, Germany; and eDepartamento de Biologia Geral, Universidade Federal de Viçosa, 36570-000 Viçosa, Minas Gerais, Brazil Edited by Raghavendra Gadagkar, Indian Institute of Science, Bangalore, India, and approved May 8, 2013 (received for review December 14, 2012) Polyphenism is the phenomenon in which alternative phenotypes are interacting with the hormonal system to translate environmental produced by a single genotype in response to environmental cues. An cues into offspring polyphenism remains mostly unknown (22). extreme case is found in social insects, in which reproductive queens The most striking example of polyphenism is found in insect and sterile workers that greatly differ in morphology and behavior societies (23), where a reproductive division of labor leads to the can arise from a single genotype. Experimental evidence for maternal coexistence of fertile queens and sterile workers that greatly differ effects on caste determination, the differential larval development in morphology and behavior (24, 25). Even though recent studies toward the queen or worker caste, was recently documented in revealed genetic influences on caste determination in social insects Pogonomyrmex seed harvester ants, in which only colonies with (reviewed in ref. 26), female caste fate is primarily influenced by a hibernated queen produce new queens. However, the proximate environmental factors in most species studied (27–39). In ants, mechanisms behind these intergenerational effects have remained several studies suggested that maternal factors such as tempera- elusive. We used a combination of artificial hibernation, hormonal ture or queen age may affect caste determination (40–44). How- treatments, gene expression analyses, hormone measurements, and ever, it is only recently that the first example of maternal effects on vitellogenin quantification to investigate how the combined effect of female caste polyphenism was documented experimentally (45). environmental cues and hormonal signaling affects the process of Cross-fostering of eggs between hibernated and nonhibernated caste determination in Pogonomyrmex rugosus.Theresultsshow Pogonomyrmex colonies revealed strong maternal effects on caste that the interplay between insulin signaling, juvenile hormone, and production, as only eggs produced by a hibernated queen were vitellogenin regulates maternal effects on the production of al- able to develop into queens, irrespective of the hibernation status ternative phenotypes and set vitellogenin as a likely key player of the rest of the colony (45). Such maternal effects on the caste in the intergenerational transmission of information. This study fate of the female offspring require that the hibernation triggers reveals how hibernation triggers the production of new queens changes in the queen that affect polyphenism in the offspring. in Pogonomyrmex ant colonies. More generally, it provides im- Hormones may be involved in this process in Pogonomyrmex ants, portant information on maternal effects by showing how envi- as Pogonomyrmex rugosus queen- and worker-destined eggs dif- ronmental cues experienced by one generation can translate into fered in their ecdysteroid content (45) and Pogonomyrmex barbatus phenotypic variation in the next generation. mature queens treated with juvenile hormone (JH) were recently found to produce larger workers (46). any plants and animals can express specific adaptive respon- Studies on the mechanisms regulating insect polyphenisms Mses to their environment through phenotypic plasticity, (reviewed in ref. 10) suggest that the insulin/insulin-like growth whereby a given genotype can develop into different phenotypes factor signaling (IIS), JH, and vitellogenin (Vg) pathways, known to regulate reproduction in adult insects (47–51), play predominant depending on environmental conditions (1, 2). Maternal effects, roles in modulating larval development in response to environ- through which the environmental conditions experienced by the mental cues. A well-known example illustrating the role of these mother are translated into phenotypic variation in the offspring pathways is the caste fate of the female brood (queen or worker) (3, 4), contribute to many phenotypic traits in a wide variety of in the honey bee Apis mellifera (52–58). In this species, worker- taxa (5, 6) and have important ecological and evolutionary con- triggered differences in larval diet induce changes in IIS that sequences (7, 8). Investigating the mechanisms of cross-genera- affect JH (57), possibly through the release of neuropeptides (e.g., tional transmission of information underlying maternal effects is allatostatin and allatotropin) that influence JH production by the needed to better understand the optimization of phenotypes in corpus allatum, as found in Drosophila (59). Changes in JH in turn changing environments (6) and, more generally, the evolution of affect the production of Vg (60–62), which may be involved in the life history strategies (9). process of caste determination (62, 63). Such effects of JH on Vg In many insect species, maternal effects are known to affect production, also reported in flies (64), locusts (65), and cockroaches polyphenism (3, 10), an extreme form of phenotypic plasticity (66), have been proposed to involve the action of ecdysteroids (62, characterized by the productionofalternativeanddiscrete 67–70). IIS, JH, and Vg may also play a role in the regulation of fi phenotypesfromasinglegenotype(1,11–13). Such maternal caste differentiation of larvae in ants, as caste-speci c expressions effects allow adequate responses to environmental cues such as temperature, photoperiod, nutrition, and population density in manyspecies(10).Examplesofmaternaleffectsoninsect Author contributions: R.L. and L.K. designed research; R.L., M.C., F.W., and D.O.A. per- formed research; J.E.S. and L.K. contributed new reagents/analytic tools; R.L. analyzed polyphenism include the production of sexual versus parthe- data; and R.L., M.C., and L.K. wrote the paper. nogenetic morphs in aphids (14, 15), winged versus wingless The authors declare no conflict of interest. fi morphs in rebugs (16), and dispersal versus solitary morphs in This article is a PNAS Direct Submission. locusts (17, 18). The endocrine system was found to play a role in 1To whom correspondence should be addressed. E-mail: [email protected]. the regulation of some maternal effects on insect polyphenisms This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. (19–21), but the nature of the physiological and genetic pathways 1073/pnas.1221781110/-/DCSupplemental. 11050–11055 | PNAS | July 2, 2013 | vol. 110 | no. 27 www.pnas.org/cgi/doi/10.1073/pnas.1221781110 Downloaded by guest on October 1, 2021 of genes involved in the IIS pathway were documented in Solenopsis 5.39, P < 0.001). Interestingly, when only pupae that did not receive invicta (71) and Diacamma sp.(72).Interestingly,caste-specific any treatment during larval development but were produced by differences in IIS, JH, and Vg were also documented in adult ants treated queens (thus, those collected after week 11) were consid- and bees (48, 73–78), suggesting further roles of these pathways in ered, there was also a significant difference between control and the regulation of social life (74, 79). methoprene-treated colonies in the proportion of queens produced We propose that the interplay between IIS, JH, and Vg regulates (t = 5.56, P < 0.001), showing that at least part of the observed maternal effects on caste polyphenism in ants by translating the effect of methoprene on caste determination was triggered by environmental conditions experienced by the queen during hiber- maternal effects. nation into the production of alternative phenotypes in the off- Whole-body queen samples were used to measure the expres- spring. Under this hypothesis, IIS would translate environmental sion of genes involved in the IIS pathway (two insulin-like peptide cues into changes in JH, which would, in turn, affect the amount of genes: ILP1 and ILP2), JH production (one gene coding for JH Vg in queens and in eggs, thus possibly affecting the caste fate of epoxidase: JHepox), and vitellogenesis (two Vg genes: Vg1 and the offspring (62, 63). This hypothesis makes four predictions. Vg2). The treatments significantly affected the expression of all of First, a pharmacological increase of JH in queens should mimic the the genes tested [ILP1: F(2,36) = 5.30, P = 0.01; ILP2: F(2,36) = effect of hibernation and stimulate the production of queens. 19.47, P < 0.001; JHepox: F(2,36) = 4.12, P = 0.02; Vg1: F(2,36) = Second, hibernation should affect IIS and the production of JH in 11.15, P < 0.001; Vg2: F(2,36) = 7.93, P = 0.001]. Compared with queens. Third, both hibernation and a JH increase should stimulate the control group, both hibernation and methoprene treatments the production of Vg in queens. Finally, Vg content should